Optically addressable matrix display

ABSTRACT

A matrix display comprises a matrix of optically addressable pixels (Pij). The pixels (Pij) comprise a light sensitive element (LSij) having a state depending on a brightness of control light (Lj) impinging on it. The pixels (Pij) further comprise a pixel light generating element (LGij) for generating pixel light (LMij) with a brightness depending on the state of the light sensitive element (LSij). The matrix display device comprises a sheet of transparent material (TS) positioned in front of the matrix of pixels (Pij), and a light source (LS) which couples source light into a side of the sheet of transparent material (TS). In use, the source light is substantially totally internally reflected in said sheet (TS). The control light (Lj) leaves the sheet (TS) at positions where the sheet (TS) is locally disturbed. This control light switches or changes the brightness of the pixels (Pij) at these positions, or provides touch position information.

The invention relates to an active matrix display, and a displayapparatus comprising a matrix display.

U.S. Pat. No. 6,215,462 discloses a matrix display device with aplurality of rows of pixels. The rows of the matrix display are selectedone by one. Each row is associated with a light waveguide whichtransports light generated by a first light emission element to thepixels of the row. A particular row is selected if the associated selectlight emission element produces light; all the other rows are notselected because their associated select light emission elements do notproduce light.

Each pixel comprises a series arrangement of a light sensitive elementand a pixel light emission element. A data voltage in accordance withthe image data to be displayed is supplied to the series arrangement viacolumn conductors. In the selected row of pixels, the light generated bythe select light emission element associated with the selected rowreaches the pixels of the selected row via the associated lightwaveguide. Consequently, the light sensitive elements of the pixels ofthe selected row have a low impedance, and the data voltage occurssubstantially over the pixel light emission elements of the pixels ofthe selected row. Thus, the selected row of pixels will generate anamount of light in accordance with the image data presented on thecolumn conductors which each are connected to a column of pixels. In therows which are not selected, the select light emission elements do notproduce light, and thus the impedance of the light sensitive elements ofnot selected pixels is high. For these pixels, the data voltage willsubstantially occur across the high impedance of the light sensitiveelements, and consequently, the voltage across the pixel light emissionelements will be below a threshold value such that the pixel lightemission elements will not produce light.

This optically addressable matrix display is not sensitive to on screeninput.

It is an object of the invention to provide a matrix display which issensitive to on screen input.

A first aspect of the invention provides a matrix display as claimed inclaim 1. A second aspect of the invention provides a display apparatusas claimed in claim 14. Advantageous embodiments are defined in thedependent claims.

The matrix display device in accordance with the first aspect of theinvention comprises a matrix of optically addressable pixels. The pixelscomprise a light sensitive element and a pixel light-generating elementThe light-generating element of a particular pixel produces a pixellight with a brightness that depends on the state of the light sensitiveelement of the particular pixel. The state of the light sensitiveelement depends on the amount of light impinging on it.

The matrix display further comprises a sheet of transparent material,which is positioned in front of the matrix of pixels, thus in-betweenthe matrix of pixels and the viewer. A light source couples source lightinto one or more sides of the sheet. This source light will continuouslyreflect at the top plane (directed towards the viewer) and the bottomplane (directed towards the matrix of pixels) of the sheet and thus stayin the sheet. This well-known effect is called total internalreflection. If at the outside of the top plane of the sheet, at aparticular position a disturbance is applied, the light will leave thebottom plane of the sheet at substantially this particular position. Thedisturbance may be a fingertip of the viewer, a stylus or any otherobject which has a refraction index which differs from the material ofthe sheet. Preferably, the object should not be transparent, to avoidthe light to leave the plane via the transparent object. Ideally, awhite surface which makes a good contact with the plane is used,however, a finger will do. Preferably, the sheet is of glass.

At the instant the disturbance is applied at the particular position,the light leaving the bottom plane of the sheet at this particularposition impinges on the light sensitive element of the pixel or pixelspresent at this particular position. Consequently, the matrix display issensitive to on-screen input

In an embodiment in accordance with the invention as claimed in claim 2,the matrix display comprise row electrodes and column electrodes. Aparticular one of the pixels is present between a particular rowelectrode and a particular column electrode. If light impinges on thelight sensitive element of this particular pixel, the impedancevariation of the light sensitive element can be detected between theparticular row and column electrode. It is thus possible to determinethe position of the disturbance in both the row and the columndirection.

For example, in an embodiment in accordance with the invention the lightsensitive element and the light generating element are arranged inseries to receive a drive voltage via the row and column electrodes. Thecontrol light impinging on the light sensitive elements of theparticular pixels associated with the position where the touch eventoccurs, will decrease the impedance of the associated light sensitiveelements. Due to the lower impedance, the current flowing through theseries arrangement will increase. It is thus possible to detect aposition of the touch event by detecting in which column and rowelectrodes a higher current is flowing. The drive voltage and thecurrent need actually not be high enough such that the pixel lightgenerating elements generate light. But if the pixel light generatingelements generate light this has the advantage that the user getsfeedback on the touch position detected because the associated pixelswill start emitting light. The pixels have to be reset regularly to stopproducing light, or the pixel should have no memory effect or a limitedmemory effect only. A reset of the pixels is possible by temporarilylowering the drive voltage.

In an embodiment in accordance with the invention as claimed in claim 3,the drive voltage across the pixels is selected sufficiently high suchthat the light impinging on the light sensitive elements of the pixelscauses the corresponding pixel light generating elements to producelight. Such a matrix display will indicate by producing light at thetouch position at which position the screen is touched. This isinteresting in applications like a drawing tablet on which one can drawa light emitting line. Such a drawing tablet can be used to makedrawings or to write a text. Preferably the pixels have at least atemporarily memory effect such that the drawing or text can be storedfor a particular time. It has to be noted that in this application it isnot relevant to actually determine the touch position. It is thus notrequired to detect in which rows and columns a touch event occurs.

In an embodiment in accordance with the invention as claimed in claim 4,the pixels are present between the row electrodes and a common backplaneelectrode. The column electrodes are not required because it is notrequired to determine the position of the touch event. This provides aless complex construction of the matrix display.

In an embodiment in accordance with the invention as claimed in claim 5,the pixels are present between a common front plane electrode and acommon backplane electrode. Both the row and the column electrodes arenot required because it is not required to determine the position of thetouch event. This provides a less complex construction of the matrixdisplay.

In an embodiment in accordance with the invention as claimed in claim 6,the drive voltage is supplied across a series arrangement of the pixellight generating element and an impedance element of which the impedancedepends on the state of the light sensitive element. If the drivevoltage has a sufficiently high level and the impedance of the impedanceelement is low, the pixel light generating element will generate lightbecause the drive voltage is substantially present across it. If thedrive voltage has a sufficiently high level and the impedance of theimpedance element is high, the pixel light generating element will notgenerate light because the select voltage is substantially presentacross the light sensitive element.

In an embodiment in accordance with the invention as claimed in claim 7,the light sensitive element itself is arranged in series with the pixellight-generating element. This has the advantage that a minimal amountof elements is used in a pixel, which provides a simple matrix display.It has to be noted that only simple two pole elements are required.Usually, if light impinges on it, the impedance of the light sensitiveelement is low with respect to the impedance of the pixellight-generating element, and if no light impinges on it, the impedanceof the light sensitive element is high with respect to the impedance ofthe pixel light generating element.

In an embodiment in accordance with the invention as claimed in claim 8,the pixels comprise a capacitor to obtain a memory behavior of thepixels. The memory behavior of the pixels may be important if theoptical state of the pixels should be kept for a longer time to indicatethe touch input by generating light at the touch position.

In an embodiment in accordance with the invention as claimed in claim 9,the pixels are constructed such that in a pixel a portion of the pixellight generated by the pixel light generating element reaches theassociated light sensitive element of the pixel. The light sensitiveelement is sensitive to the pixel light to obtain a feedback of theportion of the pixel light to the light sensitive element.

This optical feedback may be used to obtain a memory behavior of thepixel or to influence the memory behavior of the pixel. In the prior artU.S. Pat. No. 6,215,462, the pixels do not have a memory behavior.Because the pixels are selected row by row, the pixels of the rows willgenerate light during the relatively short select period of a row only.

This feedback may also be used to influence an intrinsic memory behaviorof a pixel caused by a capacitance of the pixel. The portion of thelight impinging on the light sensitive element is used to discharge thecapacitance, as is defined in the embodiment of the invention of claim12.

In an embodiment in accordance with the invention defined in claim 10,the light sensitive element itself is arranged in series with the pixellight-generating element. This has the advantage that the constructionof the matrix display is simple.

In an embodiment in accordance with the invention defined in claim 11, aswitching element has a main current path arranged in series with thepixel light generating element and a control electrode coupled to thelight sensitive element. This has the advantage that the impedance ofthe light sensitive element is less important If light of the pixellight generating element impinges on the light sensitive element itsimpedance changes, which causes the switching element to get a lowimpedance. Again a memory behavior of the pixel is obtained.

In an embodiment in accordance with the invention defined in claim 12, ashort light pulse suffices to charge the capacitor via the furtherswitching element. The capacitor is discharged by the light sensitiveelement which receives a portion of the pixel light from the pixel lightgenerating element.

In this manner, in response to the control light pulse, the pixel startswith a high brightness which gradually decreases. The control lightpulse occurs at a position where the sheet is temporarily touched. Thevalue of the capacitor determines the time during which the brightnessdecreases to zero. The brightness and/or duration of the data lightpulse determine the peak brightness of the pixel. The pixels have thus atemporarily memory behavior, this might be interesting when the matrixdisplay is used as a touch position detector which should be able todetect several touch events in a certain time frame, or when the drawingtablet should be self erasable. It is also possible to provide a userinterface allowing the user to indicate when the pixels should be reset.Lowering the drive voltage below a predetermined level can perform thereset.

Further, it is an advantage that the brightness of the pixel issubstantially independent on the quality of the pixel light-generatingelement if this is a (Poly) LED (light emitting diode). If the (poly)LED does not function well, it will take longer to discharge thecapacitor, and thus, the net amount of light produced is substantiallyequal.

Thus, now the intrinsic memory behavior of the pixels is influenced bythe feedback of the portion of the light generated by the pixellight-generating element which impinges on the light sensitive element.

These and other aspects of the invention are apparent from and will beelucidated with reference to the embodiments described hereinafter.

IN THE DRAWINGS

FIG. 1 shows a schematic representation of a display apparatus inaccordance with the invention,

FIG. 2 shows a schematic representation of a matrix display and a sheetof transparent material in accordance with an embodiment of theinvention,

FIG. 3 shows an embodiment of a display cell in accordance with theinvention,

FIG. 4 shows an embodiment of a display cell in accordance with theinvention, and

FIG. 5 shows an embodiment of a display cell in accordance with theinvention.

The same references in different Figs. refer to the same signals or tothe same elements performing the same function.

FIG. 1 shows a schematic representation of an embodiment of a matrixdisplay apparatus with optically addressed display cells or pixels. Thematrix display apparatus comprises a matrix of pixels Pij (P11 to Pmn)which are associated with intersections of column electrodes CEj (CE1 toCEn) and row electrodes REi (RE1 to REm). The index i indicates the rownumber, the index j indicates the column number of the matrix display.The row electrodes REi extend in the x-direction, the columns electrodesCEj extend in the y-direction. In a transposed matrix display, the x andy direction are interchanged.

A row driver SD supplies row voltages VRi to the row electrodes REi. Thecolumn driver DD supplies column voltages VCj to the column electrodesCEj. The drive voltage SVi which occurs between the row electrodes REiand the column electrodes CEj is present across the pixels Pij.

Optionally, the row driver SD may output a row detect signal RP whichindicates at which rows a touch event is detected, and the column driverDD may output a column detect signal CP indication at which columns atouch event is detected.

The sheet of transparent material, which is located on top of the matrixof pixels Pij, is not shown. This sheet may be integrated together withthe matrix of pixels Pij, or may be separately positioned in front ofthe substrate comprising the matrix of pixels Pij.

FIG. 2 shows a schematic representation of a matrix display and a sheetof transparent material in accordance with an embodiment of theinvention. A light source LS directs light into a side of thetransparent sheet TS. The light will be trapped in the sheet TS due tointernal reflection in the sheet TS. The substrate SU comprises thepixels Pij (not shown), the front electrodes FE and the back electrodesBE. By way of example only, the front electrodes FE are the rowelectrodes REi of FIG. 1 and the back electrodes BE are the columnelectrodes CEj of FIG. 1. The touch position is indicated by TP. At thisposition TP, the light trapped in the sheet TS will be able to leave thesheet TS as the coupled out light LC. The light LC will impinge on thepixel or the pixels Pij present at the touch position TP.

If the matrix display is used to determine the touch position TP, thedrive voltage SVi may have a level which does allow the state of thelight generating elements LGij (see FIGS. 3 to 5) to change dependent onwhether the light LC impinges on the corresponding light sensitiveelements LSij (see FIG. 3 or 4) or FLSij (see FIG. 5) or not. But thisis not required. When the light LC impinges on the pixels Pijcorresponding to the position where the touch event takes place, onlythe light sensitive elements LSij of these pixels will change state.Usually, the change of state is a change of the impedance of the lightsensitive elements. This change of state can be detected between thecorresponding row electrodes RE and column electrodes CE. Even if thepixel light generating elements PGij do not produce light. Consequently,the row driver SD is able to detect at which row position or positionsthe touch event occurs and to output a row detect signal RP indicatingthis position or these positions. In the same manner, the column driverDD is able to detect at which column position or positions the touchevent occurs and to output a column detect signal CP indicating thisposition or these positions. The row detect signal RP and the columndetect signal CP together determine the coordinates of the touchposition TP.

If the matrix display is used as a drawing tablet, the drive voltage SVimust have a level which does allow the state of the light generatingelements LGij to change dependent on whether the light LC impinges onthe corresponding light sensitive elements LSij or FLSij or not. At theinstant the light LC impinges on a pixel Pij, this pixel Pij startsemitting light. Consequently, the matrix display will produce light atthe touch position(s). Because detecting the touch position no longer isrequired, both the front electrodes FE and the back electrodes BE maybecome a plate electrode connected to all the pixels Pij. However, thefront plate FE has to be transparent to enable the light LC to reach thelight sensitive elements LSij of the pixels Pij. It might be moreconvenient to structure the front electrodes as row or column electrodesinstead of a plate electrode. It is also possible to structure the frontelectrodes FE and the back electrodes BE both in the row direction orboth in the column direction.

FIG. 3 shows an embodiment of a display cell in accordance with theinvention. The display cell or pixel Pij comprise a series arrangementof a pixel light generating element LGij and a light sensitive elementLSij of which an impedance depends on an amount of received light Lj.The series arrangement of the pixel light generating element LGij andthe light sensitive element LSij is arranged between the row electrodeREi and the column electrode CEj to receive the pixel voltage SVi (seeFIG. 1), or between the front electrodes FE and the back electrodes BE(see FIG. 2). The voltage on the row electrode REi is denoted by VRi,the voltage on the column electrode CEj is denoted by VCj, and the pixelvoltage SVi is the difference of the voltages VRi and VCj.

If the same pixel voltage SVi is supplied to all the pixels Pij, thestate of the pixel light generating elements LGij is determined by theintensity of the light LC. The level of the pixel voltage SVi isselected sufficiently high to allow the state of the pixel lightgenerating elements LGij to be changed dependent on whether theintensity of the light LC is high or low.

By way of example, the pixels Pij may be constructed such that the pixellight generating elements LGij produces light when the impedance of thelight sensitive elements LSij is low, and the pixel light generatingelements LGij do substantially not produce light when the impedance ofthe light sensitive elements LSij is high. Thus, a high intensity of thelight LC will cause the pixel light generating element LGij to producelight and a low intensity of the light LC will cause the pixel lightgenerating element LGij to not produce light. The light LC will have ahigh intensity at the touch position(s) TP and a low intensity atnon-touch positions.

Many constructions of the pixels Pij are possible, for example, it isalso possible to use a pixel construction as shown in FIG. 4 wherein thelight sensitive element LSij is used to switch a transistor TR1ij ofwhich the main current path is arranged in series with the pixel lightgenerating element LGij. Any other construction of the pixels Pijwherein an impedance value of an element arranged in series with thepixel light-generating element LGij depends on whether light is suppliedto the pixel will operate in the same manner.

In an embodiment in accordance with the invention in which opticalfeedback is present, a portion of the pixel light PLMij produced by thepixel light generating element LGij will reach the light sensitiveelement LSij.

The operation of the pixel Pij with optical feedback is elucidated inthe now following. The total amount of light falling onto the lightsensitive element LSij is the combination of the portion of the pixellight PLMij generated by the pixel light generating element LGij and thelight LC originating from the sheet TS.

Initially, the pixel Pij is in the off state, even if a considerablepixel voltage SVi is present across the series arrangement. The highimpedance of the light sensitive element LSij causes the pixel voltageSVi to be substantially present over the light sensitive element LSij,and thus a substantially zero voltage is present across the pixel lightgenerating element LGij.

If a particular pixel Pij should produce light due to the impinginglight LC at the touch position TP, the impedance of the light sensitiveelement LSij will become low with respect to the impedance of the pixellight generating element LGij and the pixel voltage SVi will besubstantially present across the pixel light generating element LGij.The pixel light generating element LGij will start to emit the pixellight LMij. Upon switching off the light LC (which usually occurs whenno touch event occurs anymore at the particular touch position TP atwhich the pixel Pij is present), the pixel Pij remains in the on-statesince the portion of the light PLMij generated by the pixel lightgenerating element LGij is captured by the light sensitive element LSijwhich keeps it impedance low. The pixel Pij may be switched off byreducing the select voltage SVi below a threshold value. The pixel Pijthus has an in-built memory brought about by optical feedback to thelight sensitive element LSij.

The pixel light generating elements LGij may, for example, comprisesmall lasers, LED's (light emitting diodes), OLED's (Organic LED's),PolyLED's, small incandescent lamps or fluorescent lamps, or lightgenerating elements as used in plasma displays. The light sensitiveelements may, for example, comprise LDR's (light dependent resistors),or LAS (light activated thyristors or other light activated electronicswitches).

Such an optical addressed display is inexpensive and relatively easy tomanufacture. The dimensions are easily scalable, only simple twoterminal memory elements are required, and a high lumen efficacy ispossible if used as a drawing tablet.

FIG. 4 shows another embodiment of a display cell in accordance with theinvention. The pixel light generating element LGij is arranged in serieswith the main current path of a transistor TR1ij between the rowelectrode REi and the column electrode CEj. The voltage on the rowelectrode REi is denoted by VRi, the voltage on the column electrode CEjis denoted by VCj, the drive voltage SVi is the difference of thevoltages VRi and VCj. The light sensitive element LSij is arrangedbetween the control electrode of the transistor TR1ij and the rowelectrode REi. An optional capacitor C1ij is arranged between thecontrol electrode of the transistor TR1ij and the column electrode CEj.An optional leakage resistor RLij is also arranged between the controlelectrode of the transistor TR1ij and the column electrode CEj.

If the control light LC impinges on the light sensitive element LSij,the transistor TR1ij becomes low-ohmic and the drive voltage SVi issubstantially present across the pixel light generating element LGijwhich starts emitting pixel light LMij. A portion of the pixel lightPLMij impinges on the light sensitive element LSij which thus will keepthe pixel in the on-state even when the light LC is not anymoresupplied. The pixel light generating element LGij will stop emittinglight when the select voltage SVi drops below a particular value. Thepixel light generating element LGij can also be switched off (or on)with the voltage Vi3.

The capacitor C1ij buffers the voltage on the control electrode of thetransistor TR1ij and provides a memory behavior. The resistor RLijdischarges the capacitor and thus determines the time constant of thememory.

FIG. 5 shows another embodiment of a display cell in accordance with theinvention. The pixel light-generating element LGij is arranged in serieswith the main current path of a transistor TR1ij between the rowelectrode REi and the column electrode CEj. The voltage on the rowelectrode REi is denoted by VRi, the voltage on the column electrode CEjis denoted by VCj, the drive voltage SVi is the difference of thevoltages VRi and VCj. The light sensitive element LSij is arrangedbetween the control electrode of the transistor TR1ij and the rowelectrode REi. An optional capacitor C2ij is arranged between thecontrol electrode of the transistor TR1ij and the row electrode REi. Amain current path of a transistor TR2ij is arranged between the controlelectrode of the transistor TR1ij and the column electrode CEj. A lightsensitive element FLSij is arranged between the control electrode of thetransistor TR2ij and the row electrode REi.

If a short light pulse impinges on the light sensitive element FLSij,the transistor TR2ij becomes low-ohmic and the capacitor C2ij is chargedto the drive voltage VSi. The transistor TR1ij starts conducting and thepixel light generating element LGij starts emitting pixel light LMij.The charge on the capacitor C2ij will keep the transistor TR1ijconductive. A portion of the pixel light PLMij impinges on the lightsensitive element LSij which will discharge the capacitor C2ij. Theimpedance of the transistor TR1ij will gradually increase. In thismanner, in response to the light pulse which occurs when the a touchevent takes place at the position of the pixel Pij, the pixel Pij startswith a high brightness which gradually decreases. The value of thecapacitor C2ij determines the time during which the brightness decreasesto zero. The brightness and/or duration of the light pulse determine thepeak brightness of the pixel Pij. This has the advantage that the matrixdisplay has a self-erasing effect. The touch position TP of the touchevents can be determined within a predetermined time frame, or theinformation displayed by the drawing tablet is kept during apredetermined time only, wherein the brightness of the display indicateshow long ago the touch input at a particular position occurred.

Further, it is an advantage that the brightness of the pixel Pij issubstantially independent on the quality of the pixel light generatingelement if this is a (Poly) LED (light emitting diode). If the (poly)LED does not function well, it well take longer to discharge thecapacitor C2ij, and thus, the net amount of light produced issubstantially equal.

It possible to switch off the pixel Pij with the voltage Vi3 at thecontrol electrode of the transistor TR2ij.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims.

For example, the transistors which are shown to be MOSFETS, may also bebipolar transistors. All the transistors may be of the oppositeconductivity type, the circuits have to be adapted in a manner known tothe skilled person.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. The word “comprising” does notexclude the presence of elements or steps other than those listed in aclaim. The invention can be implemented by means of hardware comprisingseveral distinct elements, and by means of a suitably programmedcomputer. In the device claim enumerating several means, several ofthese means can be embodied by one and the same item of hardware. Themere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

1. A matrix display with a matrix of optically addressable pixels (Pij),the pixels (Pij) comprising a light sensitive element (LSij) having astate depending on a brightness of control light (Lj) impinging on it,and a pixel light generating element (LGij) for generating pixel light(LMij) with a brightness depending on the state of the light sensitiveelement (LSij), the matrix display device comprising: a sheet oftransparent material (TS) positioned in front of the matrix of opticallyaddressable pixels (Pij), and a light source (LS) for coupling sourcelight into a side of the sheet of transparent material (TS), wherein inuse, the source light being substantially totally internally reflectedin said sheet (TS) and the control light (Lj) leaving said sheet (TS)where locally disturbed.
 2. A matrix display as claimed in claim 1,wherein the matrix display further comprises row electrodes (REi)extending in a row direction (x) of the matrix display and columnelectrodes (CEj) extending in a column direction (y) of the matrixdisplay, the pixels (Pij) being associated with interstices of the rowelectrodes (REi) and the column electrodes (CEj) and being sandwichedin-between the row drive electrodes (REi) and the column electrodes(CEj).
 3. A matrix display as claimed in claim 1, wherein the matrixdisplay further comprises drive electrodes (REi, CEj) and a pixel driver(SD, DD) for supplying a drive voltage (SVi) via the drive electrodes(REi, CEj) to the pixels (Pij), the drive voltage (SVi) being selectedfor allowing the brightness of the light generating element (Lgij) tochange depending on the brightness of the control light (Lj).
 4. Amatrix display as claimed in claim 3, wherein the drive electrodes (REi,CEj) comprise row drive electrodes (REi) extending in a row direction(x) of the matrix display and a backplane electrode, the pixels (Pij)being sandwiched in between the row drive electrodes (REj) and the backplane electrode.
 5. A matrix display as claimed in claim 3, wherein thedrive electrodes (REi, CEj) comprise a back plane electrode and a frontplane electrode, the pixels (Pij) being sandwiched in between the frontplane electrode and the back plane electrode.
 6. A matrix display asclaimed in claim 1, wherein the pixel light generating element (LGij)and an impedance element (LSij; TR1ij) are arranged in series, animpedance of the impedance element (LSij; TR1ij) being dependent on thestate of the light sensitive element (LSij), and wherein the matrixdisplay further comprises a pixel driver (SD, DD) for supplying a drivevoltage (SVi) to the series arrangement of the impedance element (LSij;TR1ij) and the pixel light generating element (LGij).
 7. A matrixdisplay as claimed in claim 6, wherein the impedance element (LSij;TR1ij) comprises the pixel light generating element (LGij) of the pixel(Pij).
 8. A matrix display as claimed in claim 1, wherein the pixels(Pij) comprise a capacitance (C2ij) to obtain a memory behavior.
 9. Amatrix display as claimed in claim 1, wherein the light sensitiveelement (LSij) and the pixel light generating element (LGij) arepositioned with respect to each other to enable a portion of the pixellight (PLMij) generated by the pixel light generating element (LGij) toreach the light sensitive element (LSij) to obtain a feedback of theportion of the pixel light (PLMij) from the pixel light generatingelement (LGij) to the light sensitive element (LSij).
 10. A matrixdisplay as claimed in claim 9, wherein the light sensitive element(LSij) and the pixel light generating element (LGij) of the pixel (Pij)are arranged in series, and wherein the portion of the pixel light(PLMij) reaching the light sensitive element (LSij) is sufficient forkeeping an impedance of the light sensitive element (LSij) relativelylow with respect to an impedance of the pixel light generating element(LGij).
 11. A matrix display as claimed in claim 9, wherein the pixels(Pij) further comprise a switching element (TR1ij) having a main currentpath arranged in series with the pixel light generating element (LGij),said series arrangement being coupled to the pixel driver (SD, DD) forreceiving the associated one of the select voltages (SVi), and having acontrol electrode coupled to the light sensitive element (LSij), andwherein the portion of the pixel light (PLMij) reaching the lightsensitive element (LSij) is sufficient for obtaining an impedance of theswitching element (TR1ij) being relatively low with respect to animpedance of the pixel light generating element (LGij).
 12. A matrixdisplay as claimed in claim 11, wherein the pixels (Pij) furthercomprise: a capacitor (C2ij) coupled to the control electrode of thefirst mentioned switching element (TR1ij), a further light sensitiveelement (FLSij) for receiving the data light (Lj), and a furtherswitching element (TR2ij) having a control electrode coupled to thefurther light sensitive element (FLSij) and a main current path coupledto the control electrode of the first mentioned switching element(TR1ij).
 13. A matrix display device as claimed in claim 1, wherein thelight sensitive element (LSij) is a light-dependent resistor or alight-activated switch.
 14. A display apparatus comprising a matrixdisplay as claimed in claim 1.